Hepatitis A virus (HAV) is a picornavirus with a single-strand positive sense RNA genome of approximately 7500 nucleotides. The wild-type strain of HAV grows poorly in cell culture, generally is not cytopathic, and virus yields are low. Cell culture-adapted mutants grow significantly more efficiently in cell culture and are attenuated for marmosets and chimpanzees. The single open reading frame of the genome is translated as one polypeptide which is then cleaved by the virus-encoded 3C proteinase to yield three or four P1 structural proteins, three P2 nonstructural proteins believed to participate in replication and three P3 nonstructural proteins that include the proteinase and RNA dependent RNA polymerase. Hepatitis A virus (HAV) was first visualized by members of the Laboratory of Infectious Diseases in 1973. In subsequent years members of the Hepatitis Viruses Section succeeded in adapting the HM175 strain of HAV to grow in primary African green monkey kidney cells (AGMK) and were able to construct infectious cDNA clones of the wild-type virulent virus and a cell culture-adapted attenuated mutant. These cDNA clones are the foundation for most of the molecular studies of virus growth and pathogenicity performed at NIH and, indeed, throughout the world. HAV is an enterically transmitted virus that causes acute hepatitis in man and some primates. Worldwide, only a single serotype has been identified and low levels of antibody are protective against clinical hepatitis A. Neutralizing antibodies are directed against conformational epitopes and intact viral capsids are the only source of immunizing antigen. The first licensed hepatitis A vaccine, an inactivated vaccine, was developed in collaboration with the HVS from the Section's prototype HAV strain. However a live attenuated hepatitis A vaccine will probably be necessary to control hepatitis A worldwide. Thus, our objectives are to determine the genetic basis for virulence and adaptation to cell culture of HAV and to use this information to develop a strain of HAV suitable for use as an attenuated vaccine.